Classic Filters

Scientific Filter provides three different types of filter which together emulate the vast majority of analog filters, and provide useful tools for analysis and measurement.

Accessed by: Effect > Scientific Filter...

Gale 03Jan14: ToDo-1 Also P1 for the GUI image. The GUI has been updated in source code but may still be under discussion.

• Peter 4Jan14: The name of this effect appears to be currently under discussion too, and may yet change.
• Peter 7Jan14: Martin committed the new name 'Classic Filters' in r13022 (not yet available in a nightly alpha).
• Gale 07Jan14: There is a build now, but little point replacing the image given the initialisation bug which is still unfixed. Are the controls in Butterworth type supposed to be centred or not?
• Peter 08Jan14: But are we confident that the new effect name will stick? Can we go ahead and change the name of the effect?
• Gale 08Jan14: I believe we've settled on "Classic Filter" (singular) according to the commit, but the -devel thread says "Classic Filters". I've asked on -devel. When the name is agreed there, by all means rename if you have time. I'm still catching up with the technical discussion. Thanks.

Bill 25Oct13: First cut at documenting this monster.
ToDo-1 Ready for editorial review and merciless editing.

• Peter 28Oct13: Linked to the proper image I just uploaded.
  I have no idea what this effect is for or how to use it, so will be of little help in the editing process. I think that the effect's author needs to be properly involved in the documentation process.
• Peter 20Dec13: Uploaded the revised image with the Hz labeling alignment fix that was made a while back.
• Gale 31Dec13: I made this P1. The more I look at this, the more questions that this page doesn't answer. However Steve is helping me towards more of a grasp of the technical issues so I still intend to attempt some "improvements".
  • Peter 31Dec13: +1 to the P1 - as I stated before, I think this documentation requires some serious input from the effect's author. It is unfair on the documentation community to just dump a complex piece of functionality like this and expect others to somehow magically understand it and write it up. Norm doesn't necessarily need to do the writing but he should be properly involved in the editing. TLC/devs should not imo accept a major piece of work like this into Audacity without seeing at least satisfactory draft documentation.

Graph Scale and Sliders

• Vertical Scale: This scale is in dB and shows the amount of gain (amplification above 0 dB or attenuation below 0 dB) that will be applied to the audio at any given frequency.
• Horizontal Scale: This shows the frequencies in Hz to which volume adjustments will be applied. Dragging the Scientific Filter window wider displays some additional points on the scale.
• Vertical scale sliders: By default the vertical scale reads from 0 dB to -10 dB, but these two sliders to left of the scale let you adjust the upper and lower dB values so as to change the visible range on the graph. Note that moving either slider may change the horizontal position of the 0 dB line.

Filter Type

• Butterworth: An analog Butterworth filter provides a "maximally flat" passband (ie. no ripples), the magnitude response at the cut-off frequency is -3 dB, and above (for lowpass) or below (for highpass) the cutoff frequency, the attenuation increases at approximately 6 dB per octave times the filter order (so e.g. 60 dB per octave for 10th order).
• Chebyshev Type I: Chebyshev Type I filters are similar to Butterworth filters, except that a) the magnitude response of the passband has "ripples" in it (usually small), b) at the cutoff frequency the magnitude response is equal to the ripple value, and c) above (below for highpass) the cut-off frequency, the stopband attenuation increases more rapidly, for a given filter order, than Butterworth.
• Chebyshev Type II: Chebyshev Type II filters are similar to Butterworth, including the flat passband response, except that a) at the cutoff frequency the magnitude response is equal to the ripple value, b) above (below for highpass) the cutoff frequency, the stopband attenuation increases more rapidly, for a given filter order, than Butterworth, and c) the stopband attenuation varies from infinite to the ripple value. (Here it's common to use a ripple value of 20, 30 or more dB).

Steve 5Nov13: I suspect that this will only make sense to users that already know what these filters are and do. If it is to be understandable to the average user I think it requires much more explanation. On the other hand, if the user is expected to know what these filter types are, then do we need to "explain" them at all?

• Peter 6Nov13: I suspect that this whole effect "will only make sense to users that already know what these filters are and do". It certainly flies well below my radar and that of many others too, I'm guessing. I remain unconvinced that this effect should be shipped as an above-the-line mainstream Audacity effect rather than as an optional add-on.

Subtype

• Lowpass: The filter passes low frequencies and attenuates high frequencies.
• Highpass: The filter passes high frequencies and attenuates low frequencies.

Order

Choose a value between 1 and 10. "1" - first-order filters - have the most gradual cutoff slope

Cutoff

Enter the cutoff frequency.

Steve 5Nov13: ToDo-2 Should be "cut-off" according to Connie, but the GUI says "cutoff"

Passband Ripple

• For Butterworth filters no value can be entered and any value displayed is ignored.

Steve 5Nov13:Do we need to say "why" that is? Someone is sure to ask.

• For Chebyshev Type I filters type in the acceptable amount of passband ripple. Higher values of passband ripple will also increase the cutoff slope.
• For Chebyshev Type II filters no value can be entered and any value displayed is ignored.

Stopband Ripple

• For Butterworth filters no value can be entered and any value displayed is ignored.
• For Chebyshev Type I filters no value can be entered and any value displayed is ignored.
• For Chebyshev Type II filters type in the desired amount of stopband ripple.

Steve 5Nov13: What is the "desired amount of stopband ripple"? What IS stopband ripple? Would stopband ripple ever be "desired"?

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